Sputtering, alternatively called physical vapor deposition (PVD), is the most prevalent method of depositing layers of metals and related materials in the fabrication of integrated circuits. The more conventional type of sputtering, as originally applied to integrated circuits as well as to other applications, deposits upon a workpiece a planar layer of the material of the target. However, the emphasis has recently changed in the use of sputtering for the fabrication of integrated circuits because vertical interconnects through inter-level dielectrics having the high aspect ratios now being used present a much greater challenge than the horizontal interconnects. Furthermore, the horizontal interconnects are being increasingly implemented by electrochemically plating copper into horizontally extending trenches while sputtering is being reserved for liner layers deposited onto the sidewalls in the holes in which the vertical interconnects are formed or also deposited onto the walls of the horizontal trenches.
It has long been known that sputtering rates can be increased by the use of a magnetron 10, illustrated in the schematic cross-sectional view of FIG. 1, positioned in back of a sputtering target 12. The magnetron projects a magnetic field 14 across the face of the target 12 to trap electrons and hence increase the plasma density. The magnetron 10 typically includes at least two magnets 16, 18 of anti-parallel magnetic polarities perpendicular to the face of the target 12. A magnetic yoke 20 supports and magnetically couples the two magnets 16, 18. The resultant increased plasma density is very effective at increasing the sputtering rate adjacent the parallel components of the magnetic field 14. However, as illustrated in the cross-sectional view of FIG. 2, an erosion region 22 develops adjacent the magnetic field, which brings a front surface 24 of the target 12 closer to the magnetron 10, which front surface 24 is the surface being currently sputtered. The erosion illustrated in FIG. 2 emphasizes an erosion pit adjacent the magnetron 10. In typical operation, the magnetron 10 is scanned over the back of the target 12 to produce a more uniform erosion pattern. Nonetheless, even if a target eroded to a planar surface, the fact remains that after erosion the surface of the target being sputtered is closer to the magnetron 10 than before erosion.
The target erosion presents several problems if the lifetime of the target 12 is to be maximized. First, the erosion pattern should be made as uniform as possible. In conventional planar sputtering, uniformity is improved by forming the magnets 16, 18 in a balanced, relatively large closed kidney-shaped ring and rotating the magnetron about the central axis of the target. Secondly, the erosion depth can be compensated by adjusting the spacing between the target and the wafer being sputter deposited, as disclosed by Tepman in U.S. Pat. No. 5,540,821. Futagawa et al. disclose a variant in U.S. Pat. No. 6,309,525. These schemes have primarily addressed the dependence of deposition rate on the separation between the wafer and the effective front face of the target 12. These approaches do not address how the erosion affects the magnetic enhancement of sputtering.
The erosion problem has been complicated by the need to produce a highly ionized sputter flux so that the ionized sputter atoms can be electrostatically attracted deep within high aspect-ratio holes and be magnetically guided, as has been explained for an SIP reactor by Fu et al. in U.S. Pat. No. 6,306,265, incorporated herein by reference in its entirety. The apparatus described therein uses a small triangularly shaped magnetron to effect self-ionized sputtering, taking into account three factors. First, it is advantageous to reduce the size of the magnetron in order to concentrate the instantaneous sputtering to a small area of the target, thereby increasing the effective target power density. Secondly, the concentrated magnetic field of the small magnetron increases the plasma density adjacent the portion of the target being sputtered, thereby increasing the ionization fraction of the target atoms being sputtered. The ionized sputter flux is effective at being attracted deep within high aspect-ratio holes in the wafer. However, the target erosion affects the effective magnetic field at the target face being sputtered, thereby changing the sputtering rate and the ionization fraction. Thirdly, the small magnetron makes uniform target sputtering that much more difficult. Various magnetron shapes, e.g., triangular have been used to increase the uniformity of sputtering, but their uniformity is not complete. Instead, annular troughs are eroded into the target even in the case of rotary magnetrons.
Two major operational effects are readily evident in the use of conventional rotary magnetrons, particularly small magnetrons. First, as illustrated in the plot 26 of the graph of FIG. 3, the deposition rate falls from its initial rate with target usage, here measured in target kilowatt-hours of cumulative power applied to the target since it was fresh. The target usage corresponds to both the amount of target that has been eroded since the target was put into service with a substantially planar and uneroded surface and to the number of wafers that have been deposited in a repetitive process. We believe that the decrease arises at least indirectly from the target erosion in which the target surface being sputtered is no longer optimized for the magnetic field since its separation from the magnetron is varying. The sputtering degradation can be compensated by either increasing the length or sputtering or Increasing the target power. Secondly, the non-uniformity of sputtering reduces the lifetime of the target to a number N1 at which the erosion trough maximum approaches the target backing plate or, in the case of an integral target, a minimum thickness of the target. At this point, to prevent either sputter deposition of the backing material or breakthrough of the target, the target must be discarded even though substantial target material survives away from the erosion troughs. Costs would be saved for target purchase, operator time, and production throughput if the target lifetime is increased.
Hong et al. have presented a planetary magnetron as a solution to the uniformity problem for a high-density plasma reactor in U.S. patent application Ser. No. 10/152,494, filed May 21, 2002, now published as Application Publication 2003-0217913, and incorporated herein by reference in its entirety. As illustrated in the cross-sectional view of FIG. 4, a plasma reactor 30 has a fairly conventional lower reactor including a reactor wall 32 which supports a sputtering target 34 through an adapter 36 and isolator 38 in opposition to a pedestal electrode 40 supporting the wafer 42 to be sputter deposited with the material of the target 34. A vacuum pump system 44 pumps the vacuum chamber to a level of a few milliTorr or less while a gas source 46 supplies a working gas such as argon through a mass flow controller 48. A clamp ring 50 holds the wafer 42 to the pedestal electrode 40 although an electrostatic chuck may alternatively be used. An electrically grounded shield 52 protects the reactor walls 32 and further acts as an anode in opposition to the target 34 while a DC power supply 54 negatively biases the target 34 to a few hundred volts to excite the argon working gas into a plasma. The positively charged argon ions are accelerated to the negatively biased target 34, which they strike and dislodge or sputter atoms of the target material. The sputtered atoms are ejected from the target 34 with fairly high energy in a wide beam pattern and thereafter strike and stick to the wafer 42. With sufficiently high target power and high plasma density, a substantial fraction of the sputtered atoms are ionized. Preferably, an RF power supply 58, for instance oscillating at 13.56 MHz, biases the pedestal electrode 40 through a capacitive coupling circuit 60 such that a negative DC self-bias develops on the wafer 42, which accelerates the positively charged sputter ions deep within high-aspect ratio holes being sputter coated.
According to the invention, a magnetron 70 positioned in back of the target 34 projects its magnetic field in front of the target 34 to create a high-density plasma region 72, which greatly increases the sputtering rate of the target 34. If the plasma density is high enough, a substantial fraction of sputtered atoms are ionized, which allows additional control over the sputter deposition. Ionization effects are particularly pronounced in sputtering copper, which has a high self-sputtering yield, as copper ions are attracted back to the copper target and sputter further copper. The self-sputtering allows the argon pressure to be reduced, thereby reducing wafer heating by argon ions and reducing argon scattering of copper atoms, whether ionized or neutral, as they travel from the target 34 to the wafer 42.
In the described embodiment, the magnetron 70 is substantially circular and includes an inner magnetic pole 74 of one magnetic polarization with respect to and extending along a central axis 76 of the chamber 32 as well as the target 34 and pedestal electrode 40. It further includes an annular outer pole 78 surrounding the inner pole 74 and of the opposed magnetic polarity along the central axis 76. A magnetic yoke 80 magnetically couples the two poles 74, 78 and is supported on a carrier 81. The total magnetic intensity of the outer pole 78 is substantially greater than that of the inner pole 74, for example by a factor of greater than 1.5 or 2.0, to produce an unbalanced magnetron which projects its unbalanced magnetic portion towards the wafer 42 to thereby confine the plasma and also guide sputtered ions towards the wafer 42. Typically, the outer pole 78 is composed of plural cylindrical magnets arranged in a circle and having a common annular pole piece on the side facing the target 34. The inner pole 74 may be composed of one or more magnets, preferably with a common pole piece. Other forms of magnetrons are encompassed by the invention.
The high plasma densities achieved by this configuration as well as that of Fu et al. are achieved in part by minimizing the area of the magnetron 70. The encompassing area of the magnetron 70 is typically less than 10% of the area of the target 34 being scanned by the magnetron 70. The magnetron/target area ratio may be less than 5% or even less than 2% if uniform sputtering is otherwise maintained. As a result, only a small area of the target 34 is subject to an increased target power density and resultant intensive sputtering. That is, the sputtering at any instant of time is highly non-uniform. To compensate for the non-uniformity, a rotary drive shaft 82 rotated by a drive source 84 and supporting the magnetron 70 circumferentially scans the magnetron 70 about the chamber axis 76. However, as has been described with respect to the reactor of Fu et al., the resultant annular troughs in the target may produce significant radial non-uniformity in the sputtering.
Hong et al. significantly reduce the sputtering non-uniformity by the use of a planetary scanning mechanism 90 to cause the magnetron 70 to move along a planetary or other epicyclic path over the back of the target 34 with respect to the central axis 76. Their preferred planetary gear mechanism 90 for achieving planetary motion includes, as additionally and more completely illustrated in FIG. 5, a fixed gear 92 fixed to a housing 94 and a drive plate 96 fixed to the rotary shaft 82. In the reactor of Hong et al., the housing 94 is stationary. The drive plate 96 rotatably supports an idler gear 98 which engages the fixed gear 92. The drive plate 96 also rotatably supports a follower gear 100 engaged with the idler gear 98. A shaft 102 of the follower gear 100 is fixed also to the carrier 81 so that the magnetron 70 supported on the carrier 81 away from the follower shaft 102 rotates with the follower gear 100 as it rotates about the fixed gear 92 to execute the planetary motion. Counterweights 110, 112 are fixed to the non-operative ends of the drive plate 96 and the carrier 81 to reduce bending and shimmy on the rotary drive shaft 82 and the follower shaft 102. Particularly in copper sputtering which achieves a high ionization ratio Cu+/Cu0 of sputtered copper ions, the sputter reactor 30 of FIG. 4 advantageously includes a magnetic coil or magnet ring 114 annular about the central axis 76 to guide the copper ions to the wafer 42.
Because the DC power supply 54 delivers a significant amount of power to the target 34 and a high flux of energetic ions bombard the target 34 thereby heating the target 34, it is conventional to immerse the magnetron 70 as well as the planetary mechanism 90 in a cooling water bath 116 enclosed in a tank 118 sealed to the target 34 and the fixed drive-shaft housing 94. Unillustrated fluid lines connect the bath 116 with a chiller to recirculate chilled deionized water or other cooling fluid to the bath 118.
The planetary magnetron scanning, because of its convolute path across the target 34, greatly improves the uniformity of target erosion so that the target 34 is more uniformly eroded and results in a nearly planar sputtering surface even as the target is eroded. As a result, the target utilization is greatly improved. Nonetheless, as the target 34 erodes generally uniformly, the magnetic field at its sputtering face is changing and apparently on average decreasing. The change affects the sputtering rate, which as described above has been observed to decrease. The plots presented in FIG. 3 are speculative. Actual experimental data are presented in FIG. 6. Plot 120 presents the measured deposition rate for copper in the planetary magnetron chamber of FIG. 4 having an axially fixed magnetron and with 28 kW of DC target power and 600 W of RF bias power as a function of target usage in kilowatt-hours. Plot 121 presents the deposition rate for a small axially fixed magnetron executing simple rotary motion, as described by Fu et al., with 56 kW of DC target power. Although the fall off in the simple rotary chamber is not as great in the planetary chamber, it is still significant. It is pointed out, however, that it may be advantageous to more heavily sputter the outer regions of the target 34, particularly when the target/wafer spacing is relatively small in order to compensate for the geometric effect of greater deposition at the wafer center. Such intended non-uniformity can be achieved by adjusting the length of the rotation arms in a planetary chamber or by changing the shape or radial position of the magnetron in a simple rotary chamber. Even in this case, the deposition rate decreases with target usage.
A second set of non-uniformity problems is not immediately addressed by the planetary scanning mechanism. The small area of the magnetron 70 advantageously produces a high target power density and high plasma density and hence increases sputtering rate and increases the fraction of ionized sputter atoms which are drawn deep within high aspect-ratio holes to coat the sides and bottom of via holes. However, the magnetic field and hence the plasma density depend upon the distance between the target sputtering surface and the magnetron. As a result, as the target 34 is being sputtered, even if uniformly, the plasma density is changing and hence the sputtering rate and the ionization rate upon which the via sidewall coverage depends are changing. The effect is exacerbated for a small magnetron because the gradient of the magnet field is greater. As a result, the changing magnetic field and plasma density destabilizes the process causing variation in bottom and sidewall coverages across the lifetime of the target. It has generally been accepted that the high-performance sputtering is different at the end of the lifetime of the target than at the beginning. Plot 122 in FIG. 7 shows the measured target voltage and plot 123 in FIG. 8 shows the measured mean bias voltage with respect to target usage for the axially fixed planetary magnetron with the aforementioned values of target and bias power. There is a significant rise in the target voltage and the magnitude of the bias voltage with increased sputtering. However, the bias voltage is subject to fluctuations of about ±20V with the maximum magnitude greatly increasing to about 150V at maximum usage. The instability is readily apparent from the plot 122 of FIG. 7 for target voltage and the plot 123 of FIG. 8 for bias power. The change of sputtering rate can be compensated by increasing the sputtering duration, but this does not address the sidewall coverage. In any case, the increased sputtering period decreases throughput and introduces another variable into the queuing plan. The variation in plasma density because of reduced magnetic field can be partially compensated by increasing the target power. Such power compensation however involves an ad hoc relationship which needs to be determined for each set of conditions and also reduces the ability to maximize plasma densities and sputtering rates with limited power supplies.
Halsey et al. in U.S. Pat. No. 5,855,744 show an apparatus for deforming a linear magnetron as it scans across a rectangular target. In one embodiment, multiple actuators moving shafts along multiple respective axes deform the magnetron. Mizouchi et al. in U.S. Pat. No. 6,461,485 discloses a single vertical actuator for compensating for end effects in linear scanning.
Demaray et al. in U.S. Pat. No. 5,252,194 discloses a slider mechanism for vertically moving a large magnetron to adjust the magnetic field at the front of the target.
Schultheiss et al. in U.S. Pat. No. 4,927,513 discloses a magnetron lift mechanism to control magnetic properties of sputtered layers.